In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset.
Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio. The ideal clutch timing for a power-on upshift is graphically depicted in FIG. 3, where Graph A depicts the clutch pressure or torque capacity for the on-coming (light trace) and off-going (heavy trace) clutches, and Graph B depicts the speed of the transmission input shaft. The shift is generally characterized as comprising three phases: a preparation phase, a torque phase, and an inertia phase. In the preparation phase, the on-coming clutch is filled in preparation for torque transmission, and the off-going clutch pressure is progressively reduced in preparation for disengagement. In the torque phase, the on-coming clutch gains torque capacity, and the off-going clutch loses torque capacity at a rate that matches the rate of increase in torque capacity of the on-coming clutch, but without a corresponding change in the input speed. The input speed change occurs during the inertia phase, as the on-coming clutch pressure is controlled to decelerate the input shaft, and the off-going clutch is fully released. FIGS. 4 and 5 graphically illustrate upshifts with improper timing of the off-going clutch disengagement. In FIG. 4, the off-going clutch is released during the preparation phase, before the on-coming clutch has achieved sufficient torque capacity; this allows the engine to momentarily accelerate the input shaft prior to the inertia phase of the shift, resulting in a loss of output torque which is perceived by the vehicle occupants as a momentary neutral sensation. In FIG. 5, the off-going clutch is released after the on-coming clutch has achieved sufficient torque capacity; this results in what is known as a tie-up interval during which the on-coming and off-going clutches are working in opposition, resulting in a sharp drop in output torque that is perceived by the vehicle occupants as a momentary braking sensation.
Since the relative timing of the on-coming engagement and the off-going disengagement is critical to achieving a high quality shift, it has been customary to use a uni-directional torque transmitting mechanism, such as a free-wheel clutch, to release the off-going clutch as the torque capacity of the on-coming clutch builds up during the torque phase of the shift, closely approximating the ideal timing depicted in FIG. 3. However, free-wheel clutches significantly increase the cost of a transmission, and various electronic control techniques have been developed for achieving clutch-to-clutch upshifts in which an electronic control unit controls both the on-coming clutch apply and the off-going clutch release. See, for example, the U.S. Pat. No. 5,058,460 to Hibner et al., issued on Oct. 22, 1991, and assigned to the assignee of the present invention, and the U.S. Pat. No. 5,119,695 to Milunas et al., issued on Jun. 9, 1992, and assigned to Saturn Corporation, which patents are incorporated herein by reference. Both of these patents utilize open-loop controls to release the off-going clutch in relation to the estimated or detected end-of-fill of the on-coming clutch. Additionally, Milunas et al. schedule the rate of reduction in off-going clutch pressure based on a confidence level determined by the controller. The off-going pressure is reduced at a slow rate when the confidence level is low, and at a fast rate when the confidence level is high, providing a variable degree of clutch overlap. Additionally, the input speed is monitored to detect a tie-up condition due to on-coming clutch engagement, and the off-going clutch is released if a tie-up condition is detected.